A wireless telecommunications system has been proposed in which a geographical area is divided in to cells, each cell having one or more central terminals (CTs) for communicating over wireless links with a number of subscriber terminals (STs) in the cell. These wireless links are established over predetermined frequency channels, a frequency channel typically consisting of one frequency for uplink signals from a subscriber terminal to the central terminal, and another frequency for downlink signals from the central terminal to the subscriber terminal.
Due to bandwidth constraints, it is not practical for each individual subscriber terminal to have its own dedicated frequency channel for communicating with the central terminal. Hence, techniques need to be applied to enable data items relating to different wireless links to be passed over the same frequency channel without interfering with each other. In current wireless telecommunications systems, this can be achieved through the use of a `Code Division Multiple Access` (CDMA) technique. One way to implement CDMA is through the application of a set of orthogonal codes to the data items to be transmitted on a particular frequency channel, data items relating to different wireless links being combined with different orthogonal codes from the set. A suitable set of orthogonal codes is a "Rademacher-Walsh" (RW) set of sixteen 16-bit codes. Orthogonal codes have the property that, when perfectly aligned, all codes cross-correlate to zero, thus making it possible to decode a signal to which one orthogonal code has been applied while cancelling interference from signals to which different orthogonal codes have been applied.
Signals to which an orthogonal code has been applied can be considered as being transmitted over a corresponding orthogonal channel within a particular frequency channel. Hence, considering the example of a set of 16 RW codes, 16 orthogonal channels can be created within a single frequency channel, and hence up to sixteen separate communication signals (corresponding to sixteen separate wireless links) can be transmitted simultaneously over the single frequency channel if different RW codes are applied to each communication signal.
It is known to provide a number of modem shelves within one central terminal, and for each modem shelf to employ a different frequency channel. Hence, if a central terminal has four modem shelves, and the set of 16 RW codes is employed for each frequency channel, one central terminal would be able to support wireless links with up to 60 subscriber terminals simultaneously.
However, as more subscribers subscribe to the wireless telecommunications network, it is becoming desirable to support more and more subscriber terminals from each central terminal. There are only a limited number of frequency channels that can be allocated to the wireless telecommunications system, and as it is desirable for neighbouring cells to use different frequency channels so as to reduce interference, the demand cannot be met by merely adding more modem shelves to each central terminal.